Intermolecular HF motion in ArnHF micromatrices (n=1,2,3,4): Classical and quantum calculations on a pairwise additive potential surface

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Abstract
The availability of pairwise additive ‘‘two‐body’’ potentials for van der Waals systems from near‐IR, far‐IR and microwave data permits detailed prediction of librational behavior for isolated HF chromophores solvated by successive numbers of rare gas Ar atoms. This paper describes theoretical calculations of ArnHF equilibrium structures and intermolecular HF vibrational frequencies based on an ArnHF ‘‘two‐body’’ potential energy surface developed from previously determined Ar–Ar and Ar–HF potentials. Isomeric structures are predicted from local minima on these multidimensional surfaces, and are found to be in excellent qualitative agreement with near‐IR observations of ArnHF clusters with n=1,2,3, and 4 Ar atoms. Quantum mechanical calculations are performed for the HF librational and van der Waals stretching modes against a rigid Arn frame. These pairwise additive potentials predict a strongly increasing angular anisotropy for the HF bending coordinate with number of Ar atoms (for small n), and provide predictions of HF intermolecular van der Waals bend and stretch vibrational frequencies. Fourier transform (FT)‐microwave and near‐IR data, on the other hand, demonstrate only a minor dependence of the anisotropy on n; this suggests the pairwise additive potentials may systematically overestimate the angular anisotropy for HF bending. Selected cuts through these potential surfaces indicate significant coupling between the Arn–HF stretch, Ar–Ar stretch, and Ar–Ar bending vibrations. This strong vibrational coupling indicates that a full quantum treatment of all intermolecular coordinates may be required in order to make quantitative comparison with van der Waals vibrational data. In the limit of sufficient Ar atoms to fill the first coordination sphere around the HF, the calculations indicate a nearly perfect cancellation of angular anisotropy for HF librational motion, consistent with the nearly free internal rotation of the HF observed in cryogenic Ar matrices.